4.2 Materials and Methods
4.3.1 PCMO Oxidation in Pure Water Vapor
A crystalline, approximately 50 nm thick sample was selected for this study. Initially, parts of the sample are covered by a 3 nm thick oxygen depleted surface layer of ion beam damaged amorphous PCMO (Figure 4.2a).
The thickness of this layer was measured under high-vacuum conditions, where no changes were observed on time scales of 30 min during irradiation with 300 keV electrons.
During the exposure to pure water vapor at a pressure of pH2O = 1 µbar and electron flux of9000e/(Å2s), the amorphous edges start to recrystallize (Figure 4.2b). Recrystal-lization of thin amorphous areas also affects the surfaces normal to the electron beam as indicated by the inhomogeneous image contrast, while keeping a crystalline Pbnm structure (Figure 4.2b). The corresponding Fourier transform (inset) shows that the initially sharp spots which belong to the Pbnm structure become diffuse. An increase of the pressure to pH2O = 5µbar during exposure to 5000e/(Å2s) results in minor changes of the morphology of the recrystallized areas. Figure 4.2c shows the state after a total of 20 min exposure to H2O and the electron beam (10 min in pH2O = 1µbar and 10 min in pH2O = 5 µbar, respectively). The HRTEM image reveals the existence of misoriented, recrystallized domains with a size<5nm. The corresponding Fourier transform exhibits a further circle-shaped spreading of the Pbnm spots as well as the appearance of some additional spots. These observations clearly indicate the formation of nanometer-sized crystalline domains with misorientation angles < 30◦ (see the inset of Figure 4.2c) by crystallization of the former amorphous areas of the sample surface.
FIG. 4.2 HRTEM images of a crystalline edge of the PCMO film in different water pressure environ-ments and electron fluxes. (a) Initial state in high vacuum. (b) State after 10 min inpH2O= 1µbar with electron flux of 9000e/(Å2s). (c) Same area 10 min after increasing the pressure to5 µbar in a flux of 5000 e/(Å2s). (d) Phase transition to amorphous state after enhancement of the pressure to 150 µbar at 5000e/(Å2s). The insets show the corresponding Fourier transformed images. In the FFT in (c) the initially sharp spots are blurred within an angle of α= 30◦. The dashed square in (d) marks the region used for the FFT.
The stability of the catalyst against beam-induced corrosion is only maintained for low water pressure. An increase of the pressure to pH2O = 150 µbar at 5000 e/(Å2s) results in a phase transition to an amorphous structure in electron-irradiated regions after a few minutes.
4.3 Results Figure 4.2d was recorded after high vacuum conditions were re-established. The observed processes cease as water vapor is evacuated from the microscope chamber. Thus, a recrystallization of initially amorphous edges via growth of misoriented nanocrystals requires a water pressure of ∼ 1−10 µbar whereas a pressure increase to > 100 µbar results in amorphization of the former crystalline lattice. The interface between crys-talline and amorphous material in Figure 4.2d roughly corresponds to the boundary of the electron beam profile. Post-characterization of the entire specimen confirms that such structural modifications only occur in regions exposed to the electron beam.
FIG. 4.3 Postmortem ELNES studies of the changes of characteristic loss edges after contact with water vapor. (a) O K-edge and (b) Mn L-edge. Initial state (black), after 5µbar water exposure (red), and the amorphous state after150µbar water exposure (blue).
Electron energy-loss near edge structure (ELNES) of the oxygen K-edge and the manganese L-edge were measured in the initial state, after exposure to 5 µbar water vapor and after amorphization in 150µbar water vapor (Figure 4.3).
The O K-edge (Figure 4.3a) exhibits three sub-features labeled A, B, and C, which are well-known for manganites. The pre-peak (feature A) arises from a hybridization of the O 2p with the eg majority spin states and is sensitive to carrier doping and bonding characteristics. Feature B can be attributed to hybridization with anti-bonding minority spin Mn-3d as well as Pr states. Feature C is related to hybridization with Pr, Ca,
and Mn states of higher energy. The O K-edge of the FIB/PIPS virgin sample shows a relatively small pre-peak feature (A) compared to other results on conventionally prepared manganite TEM lamellae at x ≈ 0.3 measured with TEM which are obtained in the literature. [172, 187] We attribute this to surface oxygen depletion to FIB TEM sample preparation. Oxygen vacancies are known as electron donors filling the majority eg states in manganites. The exposure to low-pressure water vapor (≤ 5 µbar) results in an intensity increase of the O K-pre-peak (A). This implies a depletion of majority eg states, i.e., an increase of hole doping due to oxygen vacancy healing. In contrast, subsequent exposure to 150 µbar water vapor results in a decrease of pre-peak intensity to roughly the initial value, thus indicating oxygen depletion during amorphization.
Figure 4.3b shows the Mn L3- and L2-edges (white lines) at 646 and 656 eV. We use the Gaussian fitting method for the determination of theI(L3)/I(L2) intensity ratio which is a measure for the Mn valence state [171] (for details see 4.6.2 in the Supporting Information).
We find an initial I(L3)/I(L2) ratio of 2.2. The resulting Mn valence state of +3.4 agrees with the nominal valence state expected from doping (+3.36). While O K-pre-edge (feature A) shows a clear increase after 5 µbar water exposure, only marginal changes of the Mn L-edge, i.e., a slight increase of the I(L3)/I(L2) intensity ratio, are visible.
In contrast, the corrosive amorphization of PCMO during exposure to 150 µbar water vapor results in stronger changes of both in the Mn L- and in the O K-edges. The Mn I(L3)/I(L2) intensity ratio increases to 2.5, indicating a decrease of the Mn valence to +2.8. Simultaneously, the intensity of the O K-pre-edge decreases, and there is a significant reduction of the (C) peak which is due to hybridization of O and A-site cations.
This change may indicate an incipient decomposition of the PCMO perovskite into other metal oxide compounds accompanied by a change in the Mn valence state and the A-site cation configuration (see 4.6.2 in the Supporting Information).
4.3 Results